We want to investigate the mechanisms responsible for dendritic spine abnormalities in Fragile X syndrome (FXS). FXS is the most common inherited cause of autism and mental retardation. A clear link between the functional and structural (increased density and length of spines) abnormalities in FXS has not been established. A very similar defect in spines has been found in a knockout mouse model of FXS. Spines in FXS resemble dendritic filopodia, which are spine precursors. We show that in developing mouse neocortical neurons, filopodia are replaced by spines in the second postnatal week. Interestingly, the greatest differences in dendritic protrusions between wild type and fragile X mice occur at 1 week of age, and diminish thereafter. It is conceivable that anomalies of filopodia in the first postnatal days are even more striking in the knockout mice, but this has not been explored. Our preliminary data also reveal that dendritic protrusions are longer and more densely packed when neuronal activity is blocked, so it is possible that spontaneous activity is reduced in FXS. Fragile X mice exhibit excessive group I metabotropic glutamate receptor (mGluR)-mediated long-term depression. But a direct link between abnormal mGluR signaling and spine dysgenesis has not yet been discovered. Here, we show that filopodia elongate in response to glutamate and note that others have shown that spines elongate with stimulation of group I mGluRs. We want to test the general hypothesis that a defect in filopodia, linked to abnormal group I mGluR signaling and/or to decreased neuronal activity occurs in FXS, and might impair their ability to mature into spines. Innovative and cutting-edge microscopy techniques will be used. First, we will look for abnormalities of filopodia in pyramidal neurons of fragile X mice with in vivo two-photon imaging in the first postnatal days. Next, we will examine whether spontaneous neuronal activity is reduced in neonatal fragile X mice, using two-photon calcium imaging of hundreds of neurons simultaneously. Finally, we will use two-photon glutamate uncaging to study whether glutamate-mediated elongation of filopodia is disrupted in FXS and whether mGluRs participate in this phenomenon. The experiments in this proposal are designed to identify novel molecular targets for therapeutics in FXS. Because spine abnormalities are common to several other types of mental retardation and autism disorders, these studies are of broad clinical significance.